Apparatus for conversion of sodium sulphide to sodium sulphite



J, E. GREENAWALT TUS FOR CONVE sept. 13, 1955 2,717,825 APPARA RsIoN oF soDIUM SULPHIDE TO SODIUM SULPHITE Orlglnal F'lled May 26, 1948 2 Sheets-Sheet l l IN V EN TOR. *L* fireman/alf Sept. 13, 1955 1 E GREENAWALT 2,717,825

APPARATUS FOR CONVERSION OF SODIUM SULPHIDE To SODIUM SULPHITE Orlglnal Flled May 26, 1948 2 Sheets-Sheet 2 .United States Patent() APPARATUS FOR CONVERSION OF SODIUM SULPHIDE TO SODIUM SULPHITE .lohn E. Greenawalt, Bronxville, N. Y.

Original application May 26, 1948, Serial No. 29,204. Divided and this application August 1, 1952, Serial No. 302,157

s claims. (Cl. 23-262) This invention relates to the production of the sulphites of alkali metals and apparatus.

Although there are many recognized uses for sodium sulphite, one that is particularly important is its use in the paper manufacturing industry.

principal ingredients in the cooking liquor. By use of the sulphite cooking liquor, a greater yield of pulp bers qualities is obtained than bythe use of caustic soda and sodium sulphide are the principal ingredients in the cooking liquor. This is believed to be due to the fact that sodium sulphite is much milder in its action, and it dissolves only relatively small portions such as caustic soda and sodium sulphide. But, whatever be the explanation, the use of sodium sulphite in the paper manufacturing industry would, I believe, be

claimed in my said copending application S. N. 29,204'

and said apparatus being claimed in this application.

burned and smelted to produce sodium sulphide from the sulphur compounds present. Or sodium sulphide may be obtained from other sources. Also, it has heretofore been suggested that the sodium sulphide be converted to sodium sulphite. One method that has been suggested is the conversion of hydrous sodium sulphide-to sodium 2,717,825 Patented Sept. 13, 1955 sulphite, the conversion being accomplished in accordance with the following reactions:

It will be seen that with such a process the conversion sulphate (NazSzOs). Consequently the conversion is practically all accomplished by direct oxidation of the sodium sulphide to sulphite according to the following reaction:

While my process of converting sodium sulphide to sodium sulphlte is adapted for use irrespective of the source liquor. ducing ing liquor so that it can be reused in the form of sodium sulphite. In some instances the practice has been to waste the cooking liquor.

to drop this molten mass into a tank of water where it is dissolved to make an aqueous. solution. With my process, this is not necessary. Instead, the molten mass containing sodium sulphide and sodium carbonate is retained in dry solid state and its tem.- perature reduced. This reduction ofthe temperature of the molten mass should be carried out quickly. When it is sufficiently cooled it is then treated in dry state with air to oxidize the sodium sulphide to sulphite. This is done by pulverizing the mass and treating the comminuted mass in an oxidizing atmosphere. This pulveri'zing may be done in a ball mill, a rod mill, hammer mill, disc grinder or other machine capable of fine grinding. The oxidizing is done with air and under controlled conditions. The oxidation may, if desired, be carried out with as little as one or two percent of thiosulphate being formed while the amount of sulphite produced may be as much as 50% to 60%, the balance being substantially all sodium carbonate. A finished product may be produced in which about 90% of the sulphur compounds present is in the form of sodium sulphite.

One of the diculties encountered in the direct oxidation of sodium sulphide is the tendency of the sulphide itself to burn or oxidize to the sulphate form, tion reaction being exothermic and sodium sulphide is a highly combustible compound and may flash into iiame at a low temperature. When this occurs the heat of combustion of the particles is so great that oxidation proceeds to the sulphate form. Notwithstanding` the low temperature of combustion of sodium sulphide, I have demonstrated that if the exothermic heat due to the oxidation of the sodium sulphide is controlled according to my process, the temperature of the material being treated can be as high as 250 C. without the formation of any material amount of sodium sulphate. To obtain this desirable control, I prefer to use steam under pressure in a jacket about the zone or chamber in which the material is oxidized inasmuch as a very close and fine control can be had in this manner. By adjusting the jacketed steam pressure its temperaturey can be correspondingly controlled. Hence, by maintaining the jacketed steam at a predetermined pressure the temperature can be maintained fixed so that the temperature of the material in the chamber, when the desired oxidation is carried out, and the temperature of the air used for the oxidation can be nicely controlled; allowance being made for the exothermic heat of oxidation of the sulphide. Although I have mentioned jacketed steam under controlled pressure, it will be understood that hot water under steam pressure may be circulated in the jacket as the water in a closed system under steam pressure may be considered as having a temperature cor-y responding to steam pressure.

The process may be carried out as a continuous one, the source material containing the sodium sulphide being fed, pulverized, and then progressively oxidized and continuously discharged in the sulphite form of reuse in the pulp cooking liquor or for such use ask may be desired.

Although the novel features which are believed to be characteristic of the invention will be pointedl out in the annexed claims, the invention itself as to its objects and advantages and common practice the manner 1n which it may be carried out may be better understood by reference tothe following description taken in connection with the accompanying drawings forming a part hereof, in which:

Fig. l is a view in elevation, partly in section, of apparatus embodying my invention',

Fig. 2 is a view on line 2-2 of Fig. 1; and- Fig. 3 is a view in cross-section, tol larger scale, of the rod mill to illustrate the manner of pulverizing the material to be oxidized.

The drawings illustrate an apparatus in which my process may be carried out for converting sodium sulthe oxidaphide to sodium sulphite when the source material is the product recovered from the burning of concentrated residual black liquor from the pulp digesters of a paper mill. It will be understood that there are other sources of sodium sulphide and that the description here given is for purposes of illustrating one embodiment of my invention which may be used for carrying out my process when the product resulting from burning black liquor is discharged from the furnace as a molten mass containing sodiurn sulphide and sodium carbonate. Such a furnace and a method of burning the black liquor is disclosed in my copending application Serial No. 641,995. In that apparatus the black liquor is burned in a reducing atmosphere and the sulphur compounds of the black liquor are recovered as sodium sulphide mixed with sodium carbonate. The product of the furnace is discharged as a molten owing mass from a discharge spout which is illustrated in the drawings forming a part of this application and designated by reference number 10; the furnace being shown conventionally by dotted lines.

Spout 10 leads into a cooling chamber 11 where the hot mass discharged from the spout is quickly cooled. This chamber 11 comprises an elongate water jacketed cylinder made of an inner wall 12 and an outer concentrically mounted wall 13 forming a jacketed space 14 through which to circulate a cooling medium, such as water. The jacket 14 is provided with an inlet pipe 15 with a control valve 16 and an outlet pipe 17 provided with a valve 18.

The inlet end of the jacketed cooling chamber 11 is provided with an intercommunicating receiving box 20 into which extends a steam jet pipe 21 provided with a nozzle 22 positioned to throw a jet of steam or water, if desired, into the stream of molten smelted product 23 flowing from the spout 10. The steam has a decided cooling effect upon the molten mass which may have a temperature as high as 1600'o F., and disintegrates it as it f-alls into the cooling zone 19 of cooling chamber 11. Of course if water is used it will be converted to steam by the heat of the hot smelt.

The cooling chamber is also provided with a rotatable screw conveyor 24 extending throughout its length. Its shaft 25 is mounted to rotate in bearings 26 and 27 mounted on suitable frame members 28 which are structural steel members tied together to carry the weight of the apparatus. The screw conveyor 24 is rotated by means of a prime mover. As shown, the electric motor Si) is connected to the shaft by a driving coupling 81. Communicating with the interior of cooling chamber 19 at its discharge end is a discharge or receiving box 29. This box connects at its lowerend with apparatus to cornminute or grind the cooled sodium sulphide product which has been broken up during its passage throughv the cooling zone 19 of the cooling chamber 11. This box 29 serves as a transfer chamber or conduit from the cooling chamber 11 to the grinding apparatus.

Connected to the upper end of the transfer box 29 is a conduit 30- provided with an exhauster or blower 31 operated by a prime mover (not shown). The blower 31 is commonly called an induced draft fan. The conduit 30 is connected to a dust collector 32 which serves to separate particles of solid product which may be carried over from the apparatus by the air stream which is mentioned hereinafter.

The broken up particles of sodium sulphide product are deliveredA from the cooling chamber 19 through the transfer box. 29 into the grinding apparatus which in this case is a rotatable rod mill 33. Although I prefer a rod mill, other types of comminuting or grinding apparatus, such as a ball mill, hammer mill, or disc grinder, will suggest themselves in the light of the present disclosure, it being significant to note that suitable means should be provided to finely pulverize the source material containing the sodium sulphide.

As shown the rod mill 33 comprises. an elongate rotata- 34. It will be noted that the transfer At the inlet end the supporting rollers 36 ride in a recessed ring track 37 secured to the cylinder 34 and at the discharge end the rollers 36 engage a ring track 38. The rod mill is driven by a prime mover. As shown, electric motor 39 (see Fig. 2) drives the pinion gear 40 which en gages a ring gear 41 (see Fig. 1) fixed to the cylinder box 29 outlet end has a collar 42 into A packing gland 43 is provided to make the connection substantially airtight.

As shown in Fig. 3 the interior of the rod mill is provided with a number of spiral shaped guide bales 50 and free falling rods 51. Thus when the rod mill is rotated in the direction of arrow 52 the rods 51 serve to grind the broken up pieces of sodium sulphide product. And it is significant to note that as this material works its way as it is being progressively pulverized and as it passes through the rod mill.

mill under con trolled conditions of temperature. The lower end of the transfer box 53 1s connected to the tionally) to recover any tine particles of material which may escape from the dust collector, Residual air and gases can then be passed out through a stack (not shown).

The oxidizing cylinder provided with 64 forming a jacketed a concentrically space 65 for the an oxidizing chamber with a temperature controlling jacket. An inlet pipe 67 into the jacketed space 65 is provided with a valve 68 and an outlet pipe 69 is provided with a control valve 70 so that the tempera- 1s 79 connected to the driven shaft of motor 80, which is connected by coupler 81 to shaft 80 drives screw conveyor 24 in the cooling chamber 19 noted that the apparatus provides a temperature controlled oxidizing chamber through which phide or other where it encounters steam from the nozzle of jet 21. Here the srnelt is broken up, partly solidified and cooled as it is blown by the ber the molten stream is desired, water or a mlxture of water and steam may be jetted into the molten smelt through jet 21. While the charge of material is passing through the chamber 19 it is subjected to the cooling action of the water cooled walls of this chamber; the jacket 14 being cooled by circulating water.

15, passes through jacket 14, and is discharged through pipe 17.

Acontinuously exposed to the oxidizing action of the air current as the material slowly works its way through the rod mill. desired amount and temperature in introduced through conduit 57 and ows through the rod mill countercurrent to the material. By the time it reaches the fresh material coming from the cooler i9 it is denuded of considerable oxygen and this further reduces the intensity of the oxidizing action when the material is richer in sodium sulphide. l have found that it is the too rapid oxidation of sodium sulphide particles that increases the temperature of the particles beyond that of the predetermined temperature of the oxidizing apparatus and it is this increased temperature which should be avoided because otherwise the oxidation will proceed to forni unwanted sulphate.

When'the temperature of the particles reaches about 225 C. the tendency to form sulphate is noticeable and the higher the temperature the greater is the tendency to form sulphate. An important thing 1s to control the temperature of the particles undergoing oxidation and this I may be done in my apparatus. This is accomplished by using steam as a heat transfer medium and controlling the temperature of the steam by its pressure, while at the same time constantly agitating and disturbing the particles with relation to each other while they are undergoing the oxidizing process so as to disperse the heat created by the oxidation of the sulphide particles. This control of temperature and the other conditions is done by controlling the conditions of oxidation in the oxidation chamber 54.

As the partially oxidized pulverized material is discharged from Vthe rod mill 33 it passes through transfer box 53 into the oxidation chamber 54. lt is moved through this chamber by the screw conveyor 75, which keeps the nely ground particles agitated. The temperat.

ture in the chamber is controlled by controlling the temperature of the steam passing through the jacket 65. This steam temperature is controlled by controlling its pressure. That is, it is possible to maintain the desired temperature in the oxidizing chamber by maintaining a predetermined steam pressure, the temperature being varied as desired by varying the steam pressure. I have obtained excellent results in an oxidizing chamber of this sort with a boiler pressure of 200 pounds (lbs. per square inch) which maintained a temperature about 220 C. in the oxidizing chamber. The temperature of the air introduced into this Vchamber through conduit S6 should be at about 180 C. The temperature of the air can be maintained by passing it through the air heater 87 which is heated with steam from the plant boiler. The oxidizing action may, in

some instances, be improved by mixing a small amount of steam with the air. The action of the rotating screw conveyor 75 slowly advances the material through the chamber 54 and at the same time continuously exposes new surfaces of the material to the oxidizing action of the heated air current passing countercurrent to the material. The temperature of l'he mass of pulverized particles undergoing oxidation should not substantially exceed 250 C. and should not be below about 160 C. For best results according to my present experience, the temperature in the mass of particles in the oxidizing chamber should be at about 220 C.

lThe steam for maintaining the desired temperature within the oxidizing chamber 54 enters the jacket 64 through the pipe 67 and passes out of the jacket with such water as may condense. The air passes through the chamber S4, then passes through the box 53, thence through the crushing chamber or rod mill 33 to the induced draft fan 31, which blows the residual products of the combustion and fine particles which may oat in it, into the dusty collector 32. Particles collected in the dust collector are returned through conduit 60 to the main stream in box 53 for recovery. Residual air and gases from the dust collector are led to the scrubber 62 and finally to the stack. The induced draft fan 31 also withdraws reducing gases from the furnace through the spout 10, through the cooling chamber 19, and discharges these products along with the gas stream to the collector 32 along with such residual air as results from the introduction of air through conduits 83 and 57. The treated or 'finished material is discharged from the oxidizing chamber 54 into box S2 and thence through discharge conduit S4. The recovered material contains sodium sulphite which was initially present in the charge as sodium sulphide and also sodium carbonate which was initially present. recovered product may then be dissolved in water and filtered to make up sulphite cooking liquor for the pulp digesters. After use in the digesters, the residual cooking liquor may then be recycled as described above, it being significant to note that there is no undue accumulation of sodium sulphate from recycling because sodium sulphate is reduced to sulphide in the reducing furnace.

Although the smelt coming from the furnace which makes up the charge contains other compounds beside sodium sulphide, such as sodium carbonate and possibly some carbon, these are not harmful to my process. `In fact there is an advantage because they dilute the charge so that it is not so rich in sulphide and this aids in the prevention of local heat of combustion or oxidation from forming too rapidly. The carbon, if present, is also helpful in the pulverizing step because the sodium sulphide will oxidize at a temperature below the ignition point of carbon, so that both the carbon and sodium carbonate present pass through the process unaffected.

When my process is used as a cyclic process, the recovery of chemicals will be very high, running from 90% to 95%, but,A of course, some losses will occur in the wash water and other losses due to leaks and loss through furnace gases will occur. These small losses must be replenished but this may be done very economically according to my process by adding salt Cake, or sodium sulphate, to the reducing furnace charge where it is rereduced to sodium sulphide and the sodium sulphide then converted to sodium sulphite by my oxidizing process. If it is desired to further enrich the smelt from the reducing furnace so that it will contain more sodium sulphide, this may be done by adding the desired amount of sodium sulphite to the reducing furnace. The added sodium sulphate, which may be in the form of salt cake, is reduced to sodium sulphide and discharged as such along with the smelt from the furnace.

The following examples are given further to illustrate the process of my invention.

EXAMPLE l n phite (NazSOs), 5% sodium sulphide (NaSz), 1% sodium thiosulphate (NazSrOs), 3% sodium sulphate (NazSOr), the balance being sodium carbonate. Accordingly approximately 68% of the sodium sulphide was converted to sulphite with very little formation of thiosulphate and sulphate. In the light of later experience it was found that crushing the material to greater fineness resulted in substantially higher yields of sulphite.

EXAMPLE II ln another example, a charge of molten smelt was drawn from the spout of a reducing furnace such as illustrated in the drawings. This smelt resulted from the burning of a concentrated black liquor from the digester of a paper mill. This smelt, herein called the charge, consisted of sodium carbonate, sodium sulphide, and a small Screen analysis The crushed material was then passed through the oxidizing chamber countercurrent to the preheated oxidizing air which was introduced into the oxidizing chamber.

Per cent Insoluble 2 Sodium sulphite (NazSOa) 47 Sodium thiosulphate (Na2S2O3) 2 Sodium sulphite (NazS) 4 Sodium sulphate (Na2SO4) 4 Sodium carbonate (NazCOs) 43 pulp digesters.

EXAMPLE III In another instance a charge was treated to convert its sodium sulphide to sodium sulphite. This charge was the smelt from a reducing furnace wherein waste black liquor from a pulp digester was burned. The molten charge was C. The charge was ground to a neness .of the order mentioned in Example II.

ground charge. The temperature 1n the oxidizing chamber was maintained at about 220 C. by controlling the pressure of the steam in the jacket around the oxidizing chamber.

The finished product was analyzed and found to be as follows:

Per cent Insoluble 1 Sodium sulphite (NazSOa) 27 Sodium thiosulphate (NazSzOa) 2 Sodium sulphite (NazS) l Sodium sulphate (NazSOtl) 3 Sodium carbonate (NazCOa) l63 It will be observed that of the sulphur compounds in the finished product most of it is in the form of sodium sulphite, there being only 1% of sulphide left unconverted and only 2% and 3% respectively of thiosulphate and sulphate was present, the amount of sodium sulphite being more than 80% of the sulphur compounds present.

From the foregoing disclosure it will be noted that my invention is characterized by a number of important and commercially advantageous features. It provides a process by which sodium sulphide can be converted to sodium sulphite by pulverizing solid sodium sulphide or a source material containing sodium sulphide and subjecting the pulverized particles to an oxidizing atmosphere at a temperature below that at which sodium sulphate would form l0 The process may be carried out by NazS to NazSOa without formation of undue amounts of sodium thiosulphate in the finished intermediate reaction product. It proin undue amount. direct oxidation of richer in sodium sulphite.

Not only is my process reuse over and over again.

Moreover, a novel apparatus is provided for carrying out the process wherein the cooling, grinding and oxida- This is particularly useful in a commercial plant because the plant steam boilers may conveniently, economically and advantageously be used in a simple manner to serve as a means for controlling the temperature in the oxidation process.

nized that various modications are scope of invention claimed.

What is claimed is:

l. Apparatus for converting sodium sulphide contained collected particles to said oxidizing chamber.

2. Apparatus for producing sodium sulphite from the smelt discharged from a furnace in molten state which smelt contains sodium sulphide and sodium carbonate 11 which apparatus comprises a pouring spout through which to flow the molten charge of smelt into contact with a jet of steam, `a jet mountedadjacent the discharge end of 'said spout for blowing steam into the flowing molten smelt to cool and solidify the smelt, an elongate cooling chamber the head end of whichcommunicates with said spout into which said smelt is introduced and further cooled, conveyor means within said cooling chamber operative to break up said solidified smelt and lmove it through said cooling chamber, a jacket surrounding said cooling chamber through which a cooling medium is passed, means tocontrol the ow of cooling medium through said jacket, a pulverizing chamber communicating with said cooling chamber into which the charge is passed and pulverised, a temperature controlled oxidising chamber communieating with said pulverizing chamber into which the pulverized charge is passed, a steam jacket surrounding said oxidizing chamber through which steam flows under pressure, means to regulate the steam jacket for controlling the temperature in said oxidizing chamber, conveyor means within said v,oxidizing chamber operative to agitate this pulverized charge and move it through said oxidizing chamber, `and,meansffor introducing air into said oxidizing Chamberlain it thence through said pulverizing chamber countercurrent to the passing of the charge. l

3. Apparatus for treating a molten charge containing sodium sulphide and sodium carbonate such as the smelt from a reducing furnace to convert the sodium sulphide to sodium sulphite which apparatus comprises an elongate jacketed cooling chamber in which to cool a charge of hot smelt, a pouring spout communicating with said cooling chamber through which to introduce smelt in molten condition into said cooling chamber, a steam jet mounted in said cooling chamber for introducing steam into said cooling chamber, conveyor means within said cooling chamber operative to move said charge therethrough and to break the charge into small pieces, a grinding mill having a pulverizing chamber the head end of which communicates with the discharge end of said cooling chamber, said grinding mill having means operative to progressively pulverize said pieces and discharge the pulverized charge therefrom, an elongate jacketed oxidizing chamber to receive the pulverized charge, the head end of which is connected to said pulverizing charnber and having a discharge end, conveyor means within said oxidizing chamber operative to move the pulverizedv charge discharged from said pulverizing chamber through said oxidizing chamber and discharge it therefrom, and means including a conduit for passing air into said oxidizing chamber and thence throughksaid grindingnmill'J chamber to oxidize the sodium sulphide in said charge to sodium sulphite. l t j,

4. Apparatus for producing sodium' sulphite fromthe smelt from a furnace which smelt contains sodium vsul'-` phide and sodium carbonate which apparatus comprises a spout through which to ilow Y with a jet of steam, a jet mounted adjacentthe discharge end of saidspout for blowing steam into the; flowing molten smeltrto cool and solidify the smelt, Aacooling chamber communicating with said spout into which said smelt isV introduced and further cooled, conveyor means Passing molten smelt intol contact.

pressure of the steam in said I said cooling in said cooling Vchamber operative to break the cooled smelt into small pieces and discharge them from the cooling chamber, a pulverizing chamber communicating with chamber into which the charge is passed and pulverized, grinding means in said pulverizing chamber operative to progressively pulverize said pieces, a temperature controlled oxidizing chamber communicatingwith said pulverizing chamber into which the pulverised charge is passed, and means for introducing andv passing a stream of air into said oxidizing chamber and thence through said pulverising chamber counter-current to the passage of the charge, a dust collector, a conduit connecting said dust collector with said cooling chamber and' said pulverising chamber through which said stream of air Vpasses to saiddust collector after it passes through said pulverising chamber, said dust collector serving to collect particles of the charge carried away from said cooling and pulverizing chambers in said stream of air and a conduit connecting said dust collector with said oxidizing chamber for charging the particles collected in the dust collector to said oxidizing chamber. v

5. Apparatus for treating a charge containing sodium sulphide and sodium carbonate such as the molten smelt from a reducing furnace whereby to convert the sodium sulphide to sodium sulphite which apparatus comprises an elongate cooling chamber in which to cool and solidify the molten charge of hot smelt, a jacket around said cooling chamber through which to pass a cooling fluid, means to regulate the flow of said fluid for regulating the temperature in said cooling chamber, `a screw conveyor within said cooling chamber operative to move said charge therethrough while it is being cooled, a grinding mill having a pulverizing chamber the head end of which is connected to the discharge end of said cooling chamber, said grinding mill having means operative to progressively pulverize said charge and discharge it therefrom, an elongate oxidizing chamber to receive the pulverized charge, the head end of which is connected to the discharge end of said pulverizing chamber and having a discharge end, a jacketaround said oxidizing chamber through which to pass a heated heat transfer medium, means to control the temperature of said medium, a screw conveyor within said oxidizing chamber operative to move the pulverized charge therethrough and discharge it therefrom, means including conduits for passing a stream of air into said oxidizing chamber and thence through said pulverizing chamber to oxidize the sodium sulphide in said charge toy sodiumsulphite, a dust collector communicating with said pulverizing chamber for collecting particles of charge carried by said stream of air discharged from said pulverizing chamber, and means including a conduit communicating with said oxidizing chamber for charging the collected particles into said oxidizing chamber.

References Cited in the file of this patent UNITED STATES PATENTS 

1. APPARATUS FOR CONVERTING SODIUM SULPHIDE CONTAINED COMPRISES AN ELONGATED JACKED COOLING CHAMBER IN WHICH THE MOLTEN CHARGE IS SOLIDIFIED AND COOLED, MEANS FOR INTRODUCING THE MOLTEN SMELT INTO SAID COOLING CHAMBER, SCREW CONVEYOR MEANS IN SAID COOLING CHAMBER OPIN A CHARGE OF MOLTEN SMELT TO SODIUM SULPHITE WHICH ERATIVE TO BREAK THE SOLIDIFIED CHARGE INTO PIECES AND DISCHARGE THEM THEREFROM, A MILL PROVIDING A CHAMBER INTO WHICH THE SOLIDIFIED PIECES CONTAINING SODIUM SULPHIDE ARE INTRODUCED AND GROUND TO PROGESSIVELY PULVERIZE THEM, AN OXIDIZING CHAMBER PROVIDING AN OXIDIZING ZONE INTO WHICH SAID PULVERIZED PRODUCT IS CHARGED, A JACKET AROUND SAID OXIDIZING CHAMBER THROUGH WHICH STEAM MAY BE PASSED TO CONTROL THE TEMPERATURE IN SAID ZONE, MEANS INCLUDING AN AIR CONDUIT COMMUNICATING WITH SAID ZONE FOR INTRODUCING AIR INTO SAID ZONE, AND A CONVEYORR MEANS WITHIN SAID OXIDIZING CHAMBER TO MOVE SAID PLVERIZED PARTICLES THERETHROUGH TO STIR THE PARTICLES SO THAT FRESH SURFACE OF THE PULVERIZED PARTICLES ARE CONTINUOUSLY BROUGHT INTO INTIMATE CONTACT WITH AIR INTRODUCED INTO SAID ZONE, CONDUIT MEANS FOR CARRYING SPENT AIR FROM SAID CHAMBERS, A DUST COLLECTOR CONNECTED TO SAID LAST MENTIONED CONDUIT FOR COLLECTING ENTRAINED PARTICLES OF CHARGE CARRIED IN SAID SPENT AIR, ANOTHER CONDUIT CONNECTING SAID DUST COLLECTOR AND OXIDIZING CHAMBER FOR CHARGING THE COLLECTED PARTICLES TO SAID OXIDIZING CHAMBER. 